U.S. patent application number 13/384644 was filed with the patent office on 2012-05-10 for reactive power compensation in electrical power system.
Invention is credited to Andres Agudo Araque.
Application Number | 20120112714 13/384644 |
Document ID | / |
Family ID | 43528805 |
Filed Date | 2012-05-10 |
United States Patent
Application |
20120112714 |
Kind Code |
A1 |
Agudo Araque; Andres |
May 10, 2012 |
REACTIVE POWER COMPENSATION IN ELECTRICAL POWER SYSTEM
Abstract
A power compensation system 108 for compensating for the
reactive power requirements in an electricity system 100 is
provided. The reactive power compensation system 108 includes a
static synchronous compensation unit 202, a harmonic current
elimination unit 204 and a compensation control unit 206. The
static synchronous compensation unit 202 comprises a plurality of
static synchronous compensation modules 302 for compensating for
the reactive power in the electricity system 100. The harmonic
current elimination unit 204 includes a plurality of active
filtration modules 502 for eliminating the harmonic current
generated in the electricity system 100. The compensation control
unit 206 implements a sequential control mechanism for regulating
the operation of the static synchronous compensation modules 302
and the active filtration modules 502.
Inventors: |
Agudo Araque; Andres;
(Madrid, ES) |
Family ID: |
43528805 |
Appl. No.: |
13/384644 |
Filed: |
July 27, 2009 |
PCT Filed: |
July 27, 2009 |
PCT NO: |
PCT/ES09/70316 |
371 Date: |
January 18, 2012 |
Current U.S.
Class: |
323/210 |
Current CPC
Class: |
Y02E 40/30 20130101;
Y02E 40/22 20130101; H02J 3/01 20130101; Y02E 40/16 20130101; Y02E
40/20 20130101; Y02E 40/10 20130101; H02J 3/20 20130101; H02J
3/1842 20130101; Y02E 40/40 20130101 |
Class at
Publication: |
323/210 |
International
Class: |
H02J 3/18 20060101
H02J003/18 |
Claims
1. A reactive power compensation system 108 for reactive power
compensation in a power transmission network 100, the power
transmission network 100 comprising a wind power generation unit
102, the reactive power compensation system 108 comprising: a
static synchronous compensation unit 202, the static synchronous
compensation unit 202 comprising a plurality of static synchronous
compensation modules 302 for compensating reactive power in the
power transmission network 100; a harmonics elimination unit 204,
the harmonics elimination unit 204 comprising a plurality of active
filter modules 502 for eliminating current harmonics generated in
the power transmission network 100; and a reactive power control
unit 206 for controlling the operation of the static synchronous
compensation unit 202 and the harmonics elimination unit 204.
2. The reactive power compensation system of claim 1, wherein each
static synchronous compensation module 302 comprises a capacitor
bank 402, a capacitor bank control unit 404, and a controlled
switch 406.
3. The reactive power compensation system of claim 2, wherein each
static synchronous compensation module 302 further comprises an
inverter 408, an inverter current control unit 410, and a
transformer unit 412.
4. The reactive power compensation system of claim 1, wherein each
active filter module 502 comprises a plurality of passive filters
602, an inverter 604, an inverter current control unit 606, and a
transformer unit 608.
5. The reactive power compensation system of claim 1, wherein the
reactive power control unit 206 is configured to facilitate a
sequential control of the plurality of static synchronous
compensation modules 302 and the plurality of active filter modules
502 based on a load condition in the power transmission network
100.
6. The reactive power compensation system of claim 1, wherein the
reactive power control unit 206 comprises a microcontroller.
7. The reactive power compensation system of claim 1, wherein the
reactive power control unit 206 is configured to interface with a
SCADA system to retrieve information related to magnitude of
reactive power compensation for power transmission network 100.
8. The reactive power compensation system of claim 1, wherein the
static synchronous compensation unit 202 implements a hysteresis
band control for generating a sinusoidal current in the power
transmission network 100.
9. The reactive power compensation system of claim 1, wherein the
current distortion level at load points below full-load reaches the
same current distortion level achieved at full-load.
10. The reactive power compensation system of claim 1, wherein the
current distortion level at n.sup.th factor of nominal power supply
is same as the current distortion level at nominal power, wherein n
is the number of static synchronous compensation modules 302 in the
static synchronous compensation unit 202.
11. The reactive power compensation system of claim 1, wherein the
harmonies elimination unit 204 removes current harmonics up to
19.sup.th order.
Description
FIELD OF THE PRESENT INVENTION
[0001] The present invention relates, in general, to the field of
electrical power systems, and specifically, to a system for
reactive power compensation in an electrical power system. More
specifically, the present invention relates to a system for
reactive power compensation in an electrical power system including
a wind power generation unit.
BACKGROUND OF THE PRESENT INVENTION
[0002] With the energy crisis engulfing the world, alternative
methods of energy generation are becoming increasingly important.
Wind energy has emerged as a promising renewable energy source. The
use of wind farms to generate energy is becoming increasingly
popular in developed as well as developing economies. With
improvements in material, design, and manufacturing technologies,
the volume of commercial wind energy production has been steadily
increasing. As a consequence, wind energy has become a viable and
economic renewable energy source. However, as a consequence of
uncertainty of output in wind energy based power generation units,
the integration with power transmission grids becomes a challenging
task. One of the main problems associated with wind farms is the
variation of power supply due to the intermittent nature of the
blowing winds. The non-uniform production of power leads to
problems in power system voltage and frequency stability. Thus, an
important challenge is to implement such solutions that facilitate
integration of distributed wind energy based electrical power
generation units with power transmission grids while maintaining
power quality as well as power system stability.
[0003] An important aspect of improving power quality in an
electrical power system is reactive power compensation. Electric
power includes real power and reactive power. Reactive power is
also known as watt-less power as it does not transfer any net
energy to the load. The ratio of the real power to the reactive
power is defined as the power factor of an electrical power system.
Thus, an important challenge in electric power transmission is to
control the reactive power and keep the power factor as close to
unity as possible. Effective power factor regulation ensures that a
near constant voltage is available over a wide range of load
conditions. The energy losses in long distance power transmission
lines due to power transmission line ground impedances increase the
need for reactive power compensation. In general, effective
reactive power compensation increases the power transfer capability
of the electrical power system.
[0004] One of conventional techniques of reactive power
compensation is Static VAR Compensation (SVC). However, this
technique suffers from drawbacks of fluctuating current
characteristics under low voltage situations.
[0005] In recent years, Flexible Alternating Current Transmission
System (FACTS) has emerged as a new class of solutions for
regulating power transmission parameters in electrical power
systems. FACTS is a system comprising power electronics based
components and other static equipment that facilitate control of
one or more system parameters in an AC electrical power system.
[0006] Static Compensator (STATCOM), a member of the FACTS family
of solutions, is used for reactive power compensation in AC
transmission networks. STATCOM can act as a source or as a sink of
reactive AC power in electrical power system. Accordingly, STATCOM
is used to regulate power factor in electrical power system.
Numerous control approached have been proposed in the past.
However, these control approaches fail to satisfactorily implement
an effective reactive power compensation and harmonics elimination
solution. Moreover, currently known STATCOM implementations suffer
from drawbacks arising due to a monolithic design.
[0007] Another important aspect of reactive power compensation is
control of current harmonics generated in the electrical power
system. The power generation unit should ideally experience a
sinusoidal load with minimum harmonics distortion. However, under
certain conditions, significant magnitude of low-order harmonic
currents is generated in the electrical power system and
accordingly, the power generation unit experiences a non-sinusoidal
load, which affects the stability of the electrical power system.
In the electrical power system, various parameters can lead to the
generation of current harmonics. The important factors leading to
generation of current harmonics include non-linear loads (such as
arc furnaces and static power converters), operating conditions,
and grid impedances in the electrical power system. The presence of
current harmonics affects power quality and power system stability.
Conventionally, passive filters (LC filters) are used to eliminate
the current harmonics generated in the electrical power system. The
passive filters are designed to cancel specific harmonics generated
at the load end. However, if the current harmonics spectrum
changes, the passive filters are not able to effectively attenuate
the current harmonics. In current state of the art, a few FACTS
based current harmonics elimination systems have been proposed.
However, operation of sensitive power electronics based devices
used in a wind farm are adversely affected by the FACTS system
operating close to the wind farm. This may potentially lead to high
current distortions, which leads to windmill trips and production
losses. Accordingly, use of FACTS based current harmonics
elimination systems near wind farms is a challenging task.
[0008] In view of the above problems associated with reactive power
compensation in electrical power systems, there is a need for a
system that can effectively manage the reactive power compensation
requirements and eliminate the current harmonics generated in the
electrical power system.
SUMMARY
[0009] An objective of the present invention is to achieve
effective reactive power compensation in an electrical power
system.
[0010] Another objective of the present invention is to effectively
eliminate current harmonics in the electrical power system.
[0011] Still another objective of the present invention is to
implement an improved control strategy of the reactive power
compensation system eliminate current harmonics in the electrical
power system.
[0012] Another objective of the present invention is to achieve
fault tolerance and redundancy in the reactive power compensation
system.
[0013] In accordance with an embodiment of the present invention, a
reactive power compensation system is provided. The reactive power
compensation system comprises static synchronous compensation
(STATCOM) unit, current harmonics elimination unit, and a
compensation control unit. The static synchronous compensation unit
includes a plurality of static synchronous compensation modules for
compensating reactive power in the electrical power system. The
current harmonics elimination unit includes a plurality of active
filter modules for eliminating current harmonics generated in the
electrical power system. The compensation control unit implements a
sequential control mechanism for regulating the operation of the
static synchronous compensation unit and the current harmonics
elimination unit.
[0014] Various embodiments of the present invention offer several
advantages. The present invention implements a modular design of
the reactive power compensation system. The reactive power
compensation system is operated in accordance with a sequential
control mechanism. The present invention not only effectively
manages the reactive power compensation requirements in the
electrical power system but also effectively eliminates the current
harmonics up to 19.sup.th order harmonics under varying load
conditions. Further, as a consequence of the modular design, the
present invention provides improved fault tolerance and redundancy
in the reactive power compensation system.
BRIEF DESCRIPTION OF FIGURES
[0015] The accompanying figures, where like reference numerals
refer to identical or functionally similar elements throughout the
separate views, and which, together with the detailed description
below, are incorporated in and form part of the specification,
serve to further depict various embodiments and explain various
principles and advantages, all in accordance with the present
invention.
[0016] FIG. 1 is a schematic diagram depicting an electrical power
system 100, in which various embodiments of the present invention
may be implemented;
[0017] FIG. 2 is a block diagram depicting a reactive power
compensation system 108, in accordance with an embodiment of the
present invention;
[0018] FIG. 3 is a block diagram depicting a static synchronous
compensation unit 202, in accordance with an embodiment of the
present invention;
[0019] FIG. 4 is a block diagram depicting a static synchronous
compensation module 302, in accordance with an embodiment of the
present invention;
[0020] FIG. 5 is a block diagram depicting a current harmonics
elimination unit 204, in accordance with an embodiment of the
present invention;
[0021] FIG. 6 is a block diagram depicting an active filter module
502, in accordance with an embodiment of the present invention;
[0022] FIG. 7 is a graph depicting the total harmonics distortion
relative to the total currents in the electrical power system 100,
in accordance with an embodiment of the present invention; and
[0023] FIG. 8 is a graph depicting current hysteresis control by
reactive power compensation system 108, in accordance with an
embodiment of the present invention.
[0024] It will be appreciated that elements in the figures are
depicted for simplicity and clarity and have not necessarily been
drawn to scale. For example, the dimensions of some of the elements
in the figures may be exaggerated, relative to other elements, to
help in improving an understanding of the embodiments of the
present invention.
DETAILED DESCRIPTION
[0025] Before describing in detail the system for reactive power
compensation in an electrical power system, in accordance with
various embodiments of the present invention, it should be observed
that the present invention resides primarily in combinations of
system elements related to reactive power compensation in an
electrical power system. Accordingly, the apparatus components have
been represented, where appropriate, by conventional symbols in the
drawings, showing only those specific details that are pertinent
for an understanding of the present invention, so as not to obscure
the disclosure with details that will be readily apparent to those
with ordinary skill in the art, having the benefit of the
description herein.
[0026] In this document, the terms "comprises," "comprising," or
any other variation thereof, are intended to cover a non-exclusive
inclusion, such that a process, method, article or apparatus that
comprises a list of elements does not include only those elements
but may include other elements that are not expressly listed or
inherent in such a process, method, article or apparatus. An
element proceeded by "comprises . . . a" does not, without more
constraints, preclude the existence of additional identical
elements in the process, method, article or apparatus that
comprises the element. The term "another," as used in this
document, is defined as at least a second or more. The terms
"includes" and/or "having," as used herein, are defined as
comprising.
[0027] A reactive power compensation system for managing reactive
power compensation requirements in an electrical power system is
provided. The reactive power compensation system includes a static
synchronous compensation (STATCOM) unit, a current harmonics
elimination unit, and a compensation control unit. The static
synchronous compensation unit includes a plurality of static
synchronous compensation modules for compensating reactive power in
the electrical power system. The current harmonics elimination unit
includes a plurality of active filter modules for eliminating
current harmonics generated in the electrical power system. The
compensation control unit implements a sequential control mechanism
for regulating the operation of the static synchronous compensation
unit and the current harmonics elimination unit.
[0028] FIG. 1 is a schematic diagram depicting an electrical power
system 100, in which various embodiments of the present invention
may be implemented. The electrical power system 100 includes a
power generation unit 102, a power transmission line 104,
electrical load 106, one or more reactive power compensation
systems 108a and 108b (hereinafter individually referred to as
reactive power compensation system 108 and collectively referred to
as reactive power compensation systems 108), and a plurality of
coupling transformers 110a and 110b (hereinafter individually
referred to as coupling transformer 110 and collectively referred
to as coupling transformers 110).
[0029] The power generation unit 102 may be any commonly known
power generation utility such as hydro or a thermal power plant. In
one embodiment of the present invention, the power generation unit
102 may be based on a renewable energy source of energy; more
specifically, the power generation unit 102 may be a wind farm
generating power based on wind energy.
[0030] Electrical power from the power generation unit 102 is
transmitted to the electrical load 106 through the power
transmission line 104. Examples of the electrical load 106 include
household consumers, industries and so on.
[0031] The power transmission line 104 has inherent inductive
impedance which leads to reactive power losses during the
transmission of power. Further, most examples of the electrical
load 106 are also inductive, and thus, require reactive power for
their operation. The reactive power compensation systems 108
locally supplies the reactive power required by the power
transmission line 104 and the electrical load 106. Thus, the
reactive power is not drawn from the power generation unit 102 and
thereby, the losses in the electrical power system 100 are
reduced.
[0032] The one or more reactive power compensation systems 108 may
be connected across the power transmission lines. As shown in FIG.
1 the one or more reactive power compensation systems 108 are
connected to the power transmission line 104 through the coupling
transformers 110. The reactive power compensation systems 108 may
be connected and disconnected from the power transmission line 104
by controlling the operation of the coupling transformers 110. The
operation of the coupling transformers 110 is controlled by a
Supervisory Control and Data Acquisition (SCADA) system (not shown
in FIG. 1).
[0033] In case of long-distance power transmission lines, a number
of reactive power compensation systems 108 may be connected to the
power transmission line 104 at predefined intervals. Due to
effective reactive power compensation, the reactive power
compensation system 108 facilitates voltage regulation across the
power transmission line 104.
[0034] FIG. 2 is a block diagram depicting the reactive power
compensation system 108, in accordance with an embodiment of the
present invention. The reactive power compensation system 108
includes a static synchronous compensation (STATCOM) unit 202, a
current harmonics elimination unit 204, and a compensation control
unit 206. The static synchronous compensation unit 202 includes a
plurality of static synchronous compensation modules for
compensating reactive power in the electrical power system
(explained in conjunction with FIGS. 3 and 4). The current
harmonics elimination unit 204 includes a plurality of active
filter modules for eliminating current harmonics generated in the
electrical power system (explained in FIGS. 5 and 6). The
compensation control unit 206 implements a sequential control
mechanism for regulating the operation of the static synchronous
compensation unit 202 and the harmonics elimination unit 204.
[0035] The compensation control unit 206 is based on Integrated
Gate Bipolar Transistor (IGBT) technology and employs a
microcontroller board with specific I/O ports to control the
operation of various static synchronous compensation unit 202 and
current harmonics elimination unit 204 included in the reactive
power compensation unit 108. The compensation control unit 206
interfaces with a Supervisory Control and Data Acquisition (SCADA)
system (not shown in FIG. 2), which monitors various operational
parameters in the electrical power system 100.
[0036] FIG. 3 is a block diagram depicting a static synchronous
compensation unit 202, in accordance with an embodiment of the
present invention. The static synchronous compensation unit 202
includes a plurality of static synchronous compensation modules
302a, 302b . . . , and 302n (hereinafter individually referred to
as static synchronous compensation module 302 and collectively
referred to as static synchronous compensation modules 302). Each
static synchronous compensation module 302 is connected to the
power transmission line 104 through the coupling transformer
110.
[0037] As shown in FIG. 3, the static synchronous compensation unit
202 has a modular design. One or more static synchronous
compensation modules 302 may be activated based on present
requirements in the electrical power system 100. The operation of
each static synchronous compensation module 302 is governed by the
compensation control unit 206. The compensation control unit 206
receives information related to the present load conditions and
power factor across the power transmission line 104 from the SCADA
system. The compensation control unit 206 activates one or more
static synchronous compensation modules 302 based on the
information received from the SCADA system. The compensation
control unit 206 activates the static synchronous compensation
modules 302 in a predefined sequence based on the varying load
conditions.
[0038] The modular design of the static synchronous compensation
unit 202 provides fault tolerance and redundancy in the reactive
power compensation system 108. Thus, the reactive power
compensation system 108 exhibits an improved fault ride-through
behavior by ensuring at least partial compensation of reactive
power in case of failure of one or more static synchronous
compensation modules 302.
[0039] FIG. 4 is a block diagram depicting the static synchronous
compensation module 302, in accordance with an embodiment of the
present invention. The static synchronous compensation module 302
includes a capacitor bank 402, a capacitor bank control unit 404, a
controlled switch 406, an inverter 408, an inverter current control
unit 410, a transformer unit 412, and one or more passive filters
414.
[0040] For example, the compensation control unit 206 may activate
the static synchronous compensation module 302 through triggering
the capacitor bank control unit 404. The capacitor bank control
unit 404, in turn, closes the controlled switch 406. When connected
to the power transmission line 104, the capacitor bank 402
generates reactive power to be transmitted to the power
transmission line 104.
[0041] The inverter 408 converts the DC voltage at the capacitor
bank 402 in to a voltage of desired level in accordance with the
control signal received from compensation control unit 206. Thus,
the inverter 408 acts as a voltage source of adjustable magnitude
and phase. The inverter current control unit 410 dynamically
adjusts the phase angle between the inverter voltage and the power
transmission line voltage such that the static synchronous
compensation module 302 generates (or absorbs) the desired level of
reactive power at the point of connection to the power transmission
line 104. The transformer unit 412 is a step-down transformer to
step-down the voltage in accordance with the operational voltage of
the reactive power compensation system 108.
[0042] In accordance with an embodiment of the present invention,
the output voltage of the inverter 408 is V. The voltage of the
electrical power system 100 at the point of connection of the
static synchronous compensation module 302 is V.sub.s. The output
current of the static synchronous compensation module 302 is I
which varies in accordance with V.sub.i. The static synchronous
compensation module 302 can operate in three modes. When
V.sub.i=V.sub.s, the reactive power transfer is zero and the static
synchronous compensation module 302 neither generates nor absorbs
reactive power. When V.sub.i is less than V.sub.s, the static
synchronous compensation module 302 acts as an inductive reactance
connected to the power transmission line 104. In this mode, the
current I flows from the power transmission line 104 to the static
synchronous compensation module 302, which thus, absorbs reactive
power. In the third mode, if V.sub.s is greater than V.sub.i, the
static synchronous compensation module 302 acts as a capacitive
reactance connected to the power transmission line 104. In this
mode, the current I flows from the static synchronous compensation
module 302 to the power transmission line 104, which thus,
generates reactive power. Passive filters 414 reduce the current
harmonics at the output of the static synchronous compensation
module 302.
[0043] FIG. 5 is a block diagram depicting the current harmonics
elimination unit 204, in accordance with an embodiment of the
present invention. The current harmonics elimination unit 204
includes a plurality of active filter modules 502a, 502b . . . ,
and 502n (hereinafter individually referred to as active filter
module 502 and collectively referred to as active filter modules
502). Each active filter module 502 is connected to the power
transmission line 104 through coupling transformer 110.
[0044] Similar to the static synchronous compensation unit 202, the
current harmonics elimination unit 204 has a modular design. One or
more active filter modules 502 may be activated based on present
requirements in the electrical power system 100. The operation of
each active filter module 502 is governed by the compensation
control unit 206. The compensation control unit 206 receives
information related to the current harmonics across the power
transmission line 104 from the SCADA system. The compensation
control unit 206 activates one or more active filter modules 502
based on the information received from the SCADA system. The
compensation control unit 206 activates the active filter modules
502 in a predefined sequence based on the varying load conditions.
As a consequence, the current distortion level is maintained
approximately constant for load conditions varying within a
predefined range of operating conditions.
[0045] The modular design of current harmonics elimination unit 204
provides fault tolerance and redundancy in the reactive power
compensation system 108. Thus, the reactive power compensation
system 108 exhibits an improved fault ride-through behavior by
ensuring at least partial elimination of current harmonics in case
of failure of one or more active filter modules 502.
[0046] FIG. 6 is a block diagram depicting an active filter module
502, in accordance with an embodiment of the present invention. The
active filter module 502 includes one or more passive filters 602,
an inverter 604, an inverter current control unit 606, and a
transformer unit 608.
[0047] The passive filter 602, in conjunction with other components
of the active filter module 502, is capable of generating current
harmonics opposite to the current harmonics generated in the
electrical power system 100. The operation of the inverter 604, the
inverter current control unit 606, and the transformer unit 608 are
similar to the inverter 408, the inverter current control unit 410,
and the transformer unit 412 respectively. The active filter module
502, as described herein, is capable of eliminating harmonics up to
19.sup.th order from the electrical power system 100. The
compensation control unit 206 controls the coupling transformers
110 to connect the active filter module to the power transmission
line 104.
[0048] FIG. 7 is a graph depicting the total harmonics distortion
relative to the total currents in the electrical power system 100.
The graph shows a total harmonic distortion curve 702 in accordance
with the current state of the art and a total harmonic distortion
curve 704 in accordance with the present invention.
[0049] The percentage of harmonic currents in the electrical power
system 100 is constrained to low levels for a wide range of load
conditions. The current distortion level is approximately constant
for load conditions varying within a pre-defined operating range.
The current distortion level achieved at the nth factor of nominal
power supply is the same as the current distortion level achieved
at the nominal power, `n` represents the number of active filter
modules 502 activated in the current harmonics elimination unit
204.
[0050] As shown in FIG. 7, the current distortion level for load
conditions varying between nominal power and three-fourths of the
nominal power is confined to less than 7.5%. Similarly, for load
conditions varying between nominal power and one-half of the
nominal power, the current distortion level is confined to less
than 7.5%. Moreover, for load conditions varying between nominal
power and one-fourth of the nominal power, the current distortion
level is under 10%.
[0051] FIG. 8 is a graph depicting current hysteresis control by
the reactive power compensation system 108, in accordance with an
embodiment of the present invention. The graph shows IGBT control
signal 802, a resultant current 806, a hysteresis band upper limit
804a and a hysteresis band lower limit 804b.
[0052] The reactive power compensation unit 108 implements a
hysteresis band delimited by the hysteresis band upper limit 804a
and the hysteresis band lower limit 804b. The hysteresis band
control ensures that the resultant current 806 is nearly
sinusoidal.
[0053] While various embodiments of the present invention have been
illustrated and described, it will be clear that the present
invention is not limited to these embodiments only. Numerous
modifications, changes, variations, substitutions, and equivalents
will be apparent to those skilled in the art, without departing
from the spirit and scope of the present invention, as described in
the claims.
* * * * *